drm/rockchip: Use drm_atomic_helper_commit_tail_rpm

drm/rockchip: Use drm_atomic_helper_commit_tail_rpm

Now that we use the drm psr helpers, we no longer need to hand-roll our
atomic_commit_tail implementation. So use the helper

Changes in v2:
- None
Changes in v3:
- None
Changes in v4:
- None
Changes in v5:
- None

Link to v1: https://patchwork.freedesktop.org/patch/msgid/20190228210939.83386-6-sean@poorly.run
Link to v2: https://patchwork.freedesktop.org/patch/msgid/20190326204509.96515-5-sean@poorly.run
Link to v3: https://patchwork.freedesktop.org/patch/msgid/20190502194956.218441-11-sean@poorly.run
Link to v4: https://patchwork.freedesktop.org/patch/msgid/20190508160920.144739-12-sean@poorly.run

Cc: Zain Wang <wzz@rock-chips.com>
Cc: Tomasz Figa <tfiga@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611160844.257498-12-sean@poorly.run